Transcript Cell Membranes & Movement Across Them

Cell Membranes
&
Movement Across Them
Regents Biology
2006-2007
Cell (plasma) membrane

separate cell from its environment

cell membrane is the boundary
IN
food
sugars
proteins
fats
salts
O2
H2 O
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OUT
waste
ammonia
salts
CO2
H2 O
products
cell needs materials in & products or waste out
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Building a membrane
 How do you build a barrier that keeps
the watery contents of the cell separate
from the watery environment?
What substance
do you know that
doesn’t mix with
water?
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 FATS 
 LIPIDS 
oil & water
don’t mix!!
Lipids of cell membrane
 Membrane is made of phospholipids

phospholipid bilayer
“attracted to water”
inside cell
phosphate
lipid
“repelled by water”
outside cell
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Semi-permeable membrane
 Need to allow passage through the
membrane
 But need to control what gets in or out

membrane needs to be semi-permeable
So what needs to get across the membrane?
sugar
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aa
lipid
H 2O
salt
NH3
So how do you build a
semi-permeable membrane?
Phospholipid bilayer
 What molecules can get through directly?
*small molecules
inside cell
waste
outside cell
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lipid
salt
sugar aa
H 2O
fats & other lipids
can slip directly
through the
phospholipid cell
membrane, but…
what about other
stuff?
Permeable cell membrane
 Need to allow more material through

membrane needs to be permeable to…
 all materials a cell needs to bring in
 all waste a cell needs excrete out
 all products a cell needs to export out
“holes”, or
channels, in
cell membrane
allow material
in & out
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inside cell
outside cell
Haa
sugar
2O
waste
salt
lipid
Semi-permeable cell membrane
 But the cell still needs control

membrane needs to be semi-permeable
 specific channels allow
specific material in & out
inside cell
waste
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outside
cell
H 2O
salt
aa
sugar
How do you build a semi-permeable
cell membrane?
 channels are made of proteins

proteins both “like” water & “like” lipids
bi-lipid
membrane
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protein channels
in bi-lipid membrane
Protein channels
 Proteins act as doors in the membrane

channels to move specific molecules
through cell membrane
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Most cell membranes are composed
principally of
1.DNA and ATP
2.proteins and lipids
3.chitin and starch
4.nucleotides and amino acids
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Most cell membranes are composed
principally of
1. DNA and ATP
2. proteins and lipids
3. chitin and starch
4. nucleotides and amino acids
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Movement through the channel
 Why do molecules move through
membrane if you give them a channel?
?
?
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Molecules move from high to low
 Diffusion

move from HIGH to LOW concentration
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Diffusion
 Move from HIGH to LOW concentration
“passive transport”
 no energy needed

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Simple Diffusion
 Move from HIGH to LOW
fat
inside cell
fat
fat
fat
fat
fat
Which way
will fat move?
low

high
outside cell
fat
fat
fat
fat
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fat
fat
fat
fat
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Diffusion through a channel
 Move from HIGH to LOW
sugar sugar
sugar
sugar
inside cell sugar
sugar
low

Which way will
sugar move?
high
outside cell
sugar
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sugar
sugar
sugar
sugar
sugar
Diffusion
 Move from HIGH to LOW concentration

through membrane
 simple diffusion
 no energy needed

through a protein channel
 facilitated diffusion (with help)
 no energy needed
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high
low
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Active transport
 Cells may need molecules to move
against concentration difference

need to pump “uphill”
 from LOW to HIGH using ATP
protein pump
 requires energy
 ATP

ATP
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Transport summary
diffusion
facilitated
diffusion
active
transport
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ATP
Osmosis
Movement of Water Across
Cell Membrane
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2006-2007
Osmosis
 Osmosis

diffusion of water from HIGH concentration
of water to LOW concentration of water
 across a semi-permeable membrane
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 Normally, in the process of osmosis, the net flow
of water molecules into or out of the cell depends
upon differences in the
1. concentration of water molecules inside and
outside the cell
2. concentration of enzymes on either side of the
cell membrane
3. rate of molecular motion on either side of the cell
membrane
4. rate of movement of insoluble molecules inside
the cell
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Keeping water balance
 Cell survival depends on balancing
water uptake & water loss
freshwater
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balanced
saltwater
Managing water balance
 Balanced conditions

no difference between cell
& environment
 example: blood
 problem: none
 water flows across
membrane equally,
in both directions
 volume of cell doesn’t
change
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balanced
Managing water balance
 Freshwater

a cell in fresh water
 example: Paramecium
 problem: gains water,
swells & can burst
 water continually enters
Paramecium cell
 solution: contractile vacuole
 pumps water out of cell
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freshwater
Controlling water
 Contractile vacuole in Paramecium
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Managing water balance
 Saltwater
a cell in salt water
 example: shellfish
 problem: lose water

 plasmolysis in plants
 shrinking cell

solution: take up water
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saltwater
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